The concept behind most transducers, such as linear motors used today as solenoids and voice coils used in loudspeakers have not substantially changed since they were first developed. Though substantial progress has been made in materials, magnet technology, and refinements, from an operational perspective, voice and solenoid coils have essentially remained unchanged.
Transducers and voice coils typically work on the Lorentz Force Principle, which essentially states that if a conductor carrying current is placed in a magnetic field, a force will act upon the conductor. The magnitude of this force depends on various factors such as the number of conductors, the current, the length of the conductor and the magnetic flux density.
For example, a voice coil (consisting of a former, collar, and winding) is typically a coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it. By driving a current through the voice coil, a magnetic field is produced. This magnetic field causes the voice coil to react to the magnetic field from a permanent magnet fixed to the speaker's frame, thereby moving the cone of the speaker. By applying an audio waveform to the voice coil, the cone will reproduce the sound pressure waves, corresponding to the original input signal.
From a basic perspective, a transducer or voice coil used in speakers have the same inherent problems and energy losses as traditional linear motors (or their equivalents). For instance, because the moving parts of the speaker must be of low mass (to accurately reproduce high-frequency sounds), voice coils are usually made as light weight as possible, making them delicate. Passing too much power through the coil can cause it to overheat. Voice coils wound with flattened wire, called ribbon-wire, provide a higher packing density in the magnetic gap than coils with round wire. Some coils are made with surface-sealed bobbin and collar materials so they may be immersed in a ferrofluid which assists in cooling the coil, by conducting heat away from the coil and into the magnet structure. Excessive input power at low frequencies can cause the coil to move beyond its normal limits, causing knocking and distortion.
To varying degrees, power losses present in linear motors are also present in transducers and voice coils. These losses resistive heating of the conductors, bearing losses and windage losses. These additional losses are typically referred to as hysteresis losses, inductive kickback, counter-emf., cogging, and magnetic buffeting of permanent magnet materials. A reduction or elimination of these losses would produce a more efficient transducer.
Additionally, most existing transducers and voice coils use tight clearances. Tight clearances are needed in traditional voice coils order to make use of a Lorentz force to generate movement of the conductors. The length of the field approximates the maximum distance the voice coil conductors can move. Increasing the stroke length or increasing the output power would be advantageous if one could accomplish that without increasing input power, cost, and/or heat. However, if the stroke length of the face of the pole piece is increased, the flux density available at each conductor would not change and might actually decrease. Thus, more power would be needed to achieve the same degree of movement or output power.
Lentz's law states that a counter force or counter-emf will exist to resist this movement which is felt particularly as we increase the power input to the coil and amperage increases.
Thus, some of the major inefficiencies in transducers or voice coils may be due to:
Flux density
Stroke length Clearances
I2R losses or Power losses
Counter-EMF
Heat Transfer
What is needed, therefore, is a transducer, such as used in voice coils of loud speakers that minimizes such inefficiencies resulting in a more energy efficient device.